Passive resonator reflector



Dec. 19, 1961 w. T. HARRIS PAssIvE RESONATOR REFLECTOR Filed Nov. 4, 1958 INVENTOR Maz/ Z' HAM/.5

BY l Z ATTORNEYS fa@ @Em/:Nar

United Srat Patent Filed Nov. 4, 1958, Ser. No. 771,908 24 Claims. (Cl. 340-5) y This invention relates to acoustic energy-scattering devices and reflectors, and has particular utility for underwater applications.

Regarding rst the acoustic-reflector aspect of the invention, acoustic rellectors are frequently required for use in conjunction with projecting or receiving transducers or transducer arrays to render them unidirectional. For reasons of efficiency, acoustic reiiectors should be non-dissipative but substantially opaque to the transmission of sound. Such reflectors, therefore, serve to cause an incident beam and a reliecting beam to be superposed on the incident-or front side of the transducer or array, while effectively nullifying or shielding the `array acoustically on the rear side.

The reflectors conventionally used for underwater applications are known as boundary-discontinuity reflectors. These reflectors employ a layer of a substance having an acoustic impedance which is substantially different from that of water. The reflecting layer usually comprises materials or structures which are air or gas-filled, such as air-filled cellular rubber, cork, or compliant gasfilled metal or rubber cells. These reflectors, however, have severe limitations `caused by properties which are dependent on ambient pressure, as at various depths of immersion. Further, gas-filled structures required hermetic sealing within a skincr sealing material which is impervious ,to diffusion losses. In the absence of such sealing, the structures tend to lose their gas content and become solid when submerged for long periods of time. These limitations become even more acute when the arnbient pressures approach a few thousand pounds per square inch (p.s.i.) because the structures are compressed to such anextent that theimpedance discontinuity is no longer adequate and fails to serve its function. Even a thin-walled fiat metal enclosure, which is inflated to ambient pressure with hydrogen, becomes a very poor reflector at high pressures, such as 10,000 p.s.i., unless the thickness of the hydrogen slab is made only slightly less than an acoustic wavelength. At low frequencies, such a reector would have to be very bulky, and the problem of iniiating it to maintain its thickness at varying depths of immersion is highly complex.

Inlanother aspect, the invention provides a passive acoustic energy-scattering function, useful, for example,

for decoy yor maskingv purposes lwhen released by asub merged submarine.

`It is accordingly, an object of the invention to provide improved devices of thevcharacter indicated.

yIt; is another object to provide an `improved passive, non-dissipative; devicewhich is capablev of scattering acousticv energy over -a predetermined range of frequencies. v Y

It is also an object to provide an improved passive resonant device which lends itself 'to employment in arrayconfigurations. z .f

It is ,a further object 0fthe' invention to provide a passive-resonator structure of the character indicated and which maybe either gas-filled or liquid-filled.

It is a specific object to meet the above objects with apassive resonator and resonant arrays which are extremely simple inI construction, and which are particudependent of ambient pressure, even at extreme depths of submergence.

In accordance with one aspect of the invention, there is provided a device for scattering acoustic energy, comprising a high-Q tubular resonator having amode of vibratory motion at its resonant frequency, and adapted for operation when immersed in a medium of acoustic transmission whereby. incident acoustic energy at the resonant frequency is scattered by the vibration of the resonator.

In accordance with another aspect of the invention, there is provided an array of tubular resonators having the same resonant frequency and a mode of vibratory motion at the resonant frequency. The tubular members are spaced from each other less than a quarter wavelength in the transmitting medium, whereby at the resonant frequency the acoustic energy is efciently reected by the array.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings', wherein:

FIG.1 is a longitudinal-sectional View of a passive resonator of the invention;

FIG. 2 is a simplified perspective view of the resonator used as a parasitic reflector in combination with an electro-acoustic transducer;

FIG. 3 is a perspective View of an array of such passive resonators; and n FIG. 4 is a plot of relative sound intensity as a function of frequency, illustrating operation of the array as a reflector.

Referring first to FIGURE 1, a passive resonator is shown comprising an elongated metal tube 1 and like end masses or counterweights 2-3 aifixedto the opposite ends of the tube.Vv The end weights may be bonded to the tubular member 1 by any suitable resin, such as an epoxy-resin.

The dimensions off the tubular member 1 land the masses of end weights are so chosen that the resonator length is preferably about one-half of an acoustic wavelength in the medium (eg. water) at the resonant frequency fn. At this frequency fo, a wavelength in the metal is much greater than one-half the acoustic wavelength in the medium (eg. Water), andthe resonator beh-aves somewhat as a lumped constant, mass-andspring, axially resonant vibrator.

The diameter of the resonator may typically be in the order of one-tenth of the wavelengthin the medium. The acoustic loading of the resonator in the medium is thus largely sustained by the cylindrical surface or tube `and only to a minor extent by the ends 23. Hence,

the mechanism by which the resonator is coupled to the medium is that `of what Iterm Poissons ratio radial oscillation, .resulting from the axial motion; thisv mechanism also characterizes transducers described in my copending "application, Serial No. 508,074,'1ed May 13, 1955, now Patent No. 2,880,404. InV other words, as the resonator oscillates in an axial mode of vibration, the end masses moving axiallyin opposed-phase relation, the circumferential surface of the tubular member expands and contracts'. Duiinggaxial elongation of the tubular member, there is a concomitant elastic radial contraction. Thus, efficient interaction with rediation is achieved by 4the periodic radial contraction and expansion resulting from the axial expansion and contraction, respectively. The material `of the resonator tube 1,. the dimensions Vof, rthe resonator-and the end weights 2 and 3 are selected so that the Q of the resonator in vacuum is very high, in the order of several thousand, and in the order of 10 to 50 in water. The resonators, therefore, may be considered to be substantially non-dissipative. Typically, a resonator according to FIGURE l and suitable for underwater use may comprise an aluminum alloy tube 1, 6 inches long, ll/s inches in diameter, andv 1/16 inch thick, closed by aluminum counterweights 2 3, each `approximately 11/2 inches long and bonded to tube 1 over continuous cylindrical portions for a bonded axial length of 3A; inch; when liquid-filled and immersed, such a resonator has a resonant frequency of about 5000 c.p\.s.

Although sealed hollow resonators can be constructed to have desired acoustic properties, and still withstand very high pressure, it may be advisable, particularly for use` at great depths, to solidly fill the resonators with liquid, so that the resonator structure is not stressed by the ambient pressure. For this purpose, bleeder ports 4 and 5 may` be provided in the end weights 2 and 3, respectively. The diameter of the bleeder ports is selected so that the ports are effectively closed to the vibratory motion of the liquid but are just enough open to accommodate the slight ow accompanying changesI in ambient` pressure at varying depths of immersion. In spite of liquid filling, the liquid-filled device is a high-'Q device `and may be regarded as non-dissipative, and except for its limitations at great underwater depths, an airor gas-filled device of FIG. l is also non-dissipative and is characterized by highQ.

The single resonator has utility as an acoustic scatte-rer at its resonant or higher frequencies. Such devices, for example, may be released by submarines to serve as noise-scattering decoys or masking devices, and may be designed to be neutrally buoy-ant, or negatively or positively buoyant, depending on tactical necessity. Other uses for the acoustic-energy scatterer will be apparent to those skilled in the art,

Thesingle-element resonator 1 may also be utilized as a parasitic reflector in combination with an electroacoustic transducer 6, as shown in FIG. 2. The transducer 6 is shown diagrammatically, since the details of the transducerare unnecessary for anl understanding of the invention; it may, for example, be of the Poissonratio type disclosed in said'copending application. The transducer 6 is shown with lead-out wires 7, for suitable circuit connection. The reflector 1 is positioned, preferably, parallel to and one-quarter wavelength behind the transducer 6, so that the reflected energy is superposed on the incident radiation of the transducer, and if like tube and end-mass proportions and materials` 4are employed, both the reflector 1 and the transducer 6 may have the same resonant frequency.

Suitable support for the elements of FIG. 2 may be provided locally or at the longitudinal center of each ofthe elemental tubes 1 and 6. For the free-hooded' or liquid-filled embodiment, spaced bracket members or arms lil-11' carry rubber or plastic bushings 12--13 circumferentially embracing the supported parts of elements 1-6. The'material of bushings 12--13 preferably such as to have substantially the sound-transmitting characteristics of water so'as not to impair resonant motion of either element 1 or 6.V

A planar array of resonators spaced close together, e`.g., .less than' one-quarter wavelength, comprises a highly effective reflector of plane waves. In FIGURE 3, a planar array is illustrated, with the resonators 1 arranged vertically and parallel in a first row 14, adjacent a similar second row 15. A normally incident sound wave', as suggestedr by an arrow impinginguon one side. of the array, isreected back in the-direction of thetransducer 6; a relatively smallportionpof incident energy is'transmitted through thefarray. Support means for the individual elements of the ,arrayI are not shown but, are suggested at 16-171S `and will be understood -to` be provided in a manner analogous to that shown and described in connecvertically in the array, it is obvious that the elements may be suspended horizontally. Other array configurations will be obvious to those skilled in the art.

A complete mathematical description of the operation of the resonators or resonator array is rather complex. However, the effectiveness of an array can be qualitatively appreciated when it is realized that at frequencies below the resonant frequency of the resonators, the resonators behave as rigid bodies immersed in the medium, and hence the spaced array would be effectively transparent. However, at the resonant frequency of the resonators, the array (as in FIG. 3) tends to become effectively a compliant layer in the medium, and hence presents an impedance discontinuity which is capable of refleeting the incident sound energy. At frequencies above the resonant frequency of the resonators, the resonators still tend to present effectively a vlayer of compliant material to the incident sound waves as long as they remain closely spaced relative to a wavelength, and, as indicated above, preferably less than a quarter wavelength. Eventually, as progressively higher frequencies are considered, the reflective layer may be expected tolose its ordinarily continuous characteristic and become an array of discrete objects. At suiciently high frequencies, the purelyradial resonant mode of the cylinders will be approached, causing the reflectors to operate in a different mode of vibration and independent of motion of end masses 2-3.

ln FIG. 4 the behavior of an array of reflectors is illustrated by a plot of relative sound intensity in the medium as a function of frequency. It is seen that the array is transparent to sound at frequencies appreciably below the resonant frequency fo of the resonators. At frequency fo, the resonator array becomes a good reflector and re-v mains effective at frequencies above the resonant frequency subject to the limitation noted above for the substantially higher frequencies at which element spacings are greater than one quarter of a wavelength.

It will be apparent to those skilled in the art that passive reflectors may also serve a very valuable purpose as markers on which underwater bearings are to be taken; Such arrays may become, in effect, underwater beacons, responding to directionally responsive echo-ranging sonar equipment, and if different underwater locations are marked by such arrays resonant at unique and different frequencies, the detected range, bearing, and frequency of a particular marker can provide exact knowledge of ones underwater location.

While the foregoing description sets forth the principles of the invention in connection with specific apparatus, it is to be clearly understood that this description is' made only by way of example and not as a limitation of the scope of the invention as set forth in the objects thereof and in the accompanying claims.

I claim:

' l. A device adapted to be suspended in a medium of acoustic transmission and adapted to scatter acoustic energy of a predetermined character which impinges thereupon, said -device comprising an elongated elastic tubular resonator having a resonant frequency of vibration in a vibratory mode such that the ends of said resonator move in the direction 0f its axis and its elongated side surfaces contract and expand in a radial direction, said side surfaces being substantially completely acoustically operatively exposed to the medium in which it is suspended and substantially free to vibrate radially.

2. The device of claim l, in which the length of said device is on the order of one-half the wavelength, in said medium, ofl vibrations at said resonant frequency.

, 3. The device of claim l, in which the length of said device is on the order of one-half the wavelength, in said medlum, ofvibrations at said resonant frequency and the diameter of said device is on the order of one-fifth its length.

4. The device of claim 1, in which said device has a hollow interior and is provided withV an opening communicatingbetweensaidinterior and thev exterior thereof,

through which said medium is adapted to pass', said Opening being of such dimension as to admit the surrounding medium into the interior of said device but being edectively closed to the vibratory motion of the medium within the device.

5. The device according to claim 1, wherein said resonator comprises a hollow metal cylindrical member, and a pair fof end weights affixed to and closing the opposite ends of said cylinder.

6. The device accor-ding to claim 5, wherein said resonator has an axial llength approximately one-half of the acoustic wavelength, in said medium, of vibrations at said resonant frequency, and its diameter is substantially less than its length.

7. The device according to claim 5, wherein said device is provided with an opening communicating between the hollow interior thereof and the exterior thereof, through which said medium is adapted to pass, said opening being of such dimension as to admi-t the surrounding medium into the interior of said cylinder but being effectively closed to the vibratory motion of the medium within the cylinder.

8. The device of claim 5, and including a liquid completely lling the interior volume of said device.

9. The device of claim 5, and including a gas cornpletely filling the interior volume of said device.

`10. An acoustic-energy reector, comprising a plurality of elongated elastic tubular resonators, each having a longitudinally resonant mode of vibratory motion at a predetermined resonant frequency, and means mounting said tubular resonators to form an array, in which said resonators are parallel and in which the spacing between adjacent resonators is less than substantially one quarter the acoustic wavelength in the transmitting medium at the highest of said resonant frequencies, whereby at said resonant frequency the acoustic energy is substantially reflected by said array.

1l. The reilector according to claim 10, wherein said plurality of resonators resonate at substantially the same frequency.

12. The reflector according to claim 10, wherein each of said resonators comprises a hollow metal cylindrical member and a pair of weights of substantially equal mass aiiixed to the opposite ends of said cylindrical member.

13. The reector Vaccording to claim wherein the axial length of each of said reflectors is approximately one-half the acoustic wavelength, in the transmitting medium, at its resonant frequency, and in which the diameter of said cylinder is substantially less than the axial length thereof.

14. The reflector according to claim 10, wherein each of said reectors is filled-With liquid.

15. The resonator of claim V10, in which said resonators have side surfaces which vibratorily resonantly contract and expand in a radial direction when said resonaaixed to the opposite ends of said cylindrical member.

18. The reector according to claim 15, wherein the axial length of each of said reflectors is approximately one-half the acoustic wavelength, in the transmitting medium, at its resonant frequency, and in which the diameter of said cylinder is substantially less than the axial length thereof.

19. The reiiector according to claim 15, wherein each of said reflectors is filled with liquid.

2l). An acoustic energy reiiector, Vcomprising a plurality of tubular resonators, each resonator comprising a hollow metal cylinder and a pair of weights aiiixed to the opposite ends of said cylinder, the cylinder and end Weights lof each resonator being selected so as to determine a resonant frequency in an axial mode of vibratory motion for that resonator, the lengths of said resonators respectively being approximately equal to one-half an acoustic wavelength, in the transmitting medium at their respective resonant frequencies, the diameters of said cylinders being'swbstantially less than the axial lengths thereof, and the spacing between adjacent resonators being less than one-quarter of an acoustic wavelength, in the transmitting medium, at the highest of said resonant frequencies, whereby at said resonant frequency the acoustic energy is substantially reected by said array.

21. The resonator of claim 20, in which said resonators have side surfaces which vibratorily resonantly contract and expand in a radial direction when said resonators vibrate longitudinally, said resonators being mounted with their side surfaces substantially completely acoustically operatively exposed to the medium which surrounds them and substantially completely free to vibrate radially.

22. In combination, an electro-acoustic transducer capable of transmitting or receiving energy in water as the acoustic medium, an elongated essentially cylindrical resonator having la resonant frequency of vibration in a vibratory mode such that the ends of said resonator move in the direction of its axis and the side surfaces of said resonator contract and expand in a radial direction, and means for mounting said` transducer and said resonator with said resonator spaced from said transducer by a distance equal to NA/4, where N is anod-d integer and )t is the wavelength of sound in water at said resonant frequency, the axis of said resonator being perpendicular to a line through the centers of said transducer and resonator.

23. The combination of claim 22, in which said transducer comprises an elongated essentially cylindrical body which vibrates at a frequency and in a mode similar to said resonator, and electro-mechanical means operatively associated with said body for transducing purposes.

24. The resonator of claim 22, in which said resonator has side surfaces which vibratorily resonantly contract and expand in a radial direction when said resonator vibrates longitudinally, said resonator being mounted with its side surfaces substantiallycompletely acoustically operatively exposed to the medium which surrounds it and substantially completely free to vibrate radially.

References Cited in the le of this patent UNITED STATES PATENTS 2,063,950 Steinberger Dec. 15, 1936 2,478,207 Robinson Aug. 9, 1949 2,839,735 Van Atta June 17, 1958 2,848,672 Harris Aug. 19, 1958 

